1. Field of the Invention
The present invention relates generally to steroids, and more particularly to the stereocontrolled synthesis of side chains onto tetracyclic steroidal compounds.
2. Prior Art
Steroids are derivatives of a perhydrocyclopentanophenanthrene nucleus which has the structure (including carbon and ring numbering) illustrated below: ##STR1## Many biologically active steroids include a substituent group at C.sub.3 of the first ring and a side chain at C.sub.17 of the fourth ring. For example, sterols represent one, extremely abundant, class of steroids which include an alcoholic hydroxyl group at C.sub.3 and a branched, aliphatic chain at C.sub.17.
Among various of the naturally occurring steroid hormones are those including a carbon-carbon double bond between C.sub.5 and C.sub.6 (for example, pregnenolone) and those which include a carbon-carbon double bond between C.sub.4 and C.sub.5 (for example, progesterone).
There is considerable interest in synthesizing naturally occurring steroids, novel steroids and derivatives thereof. For example, various procedures have involved the introduction of functionalized steroidal side chains in efforts to synthesize a variety of ecdysones, vitamin D metabolites, and unusual marine sterols.
However, the various syntheses involving steroidal side chains have frequently provided low yields, and/or have involved a number of steps which have increased costs of commercial syntheses of biologically active steroids and their derivatives. Additionally, many of the known efforts towards introducing steroidal side chains have not been stereospecific. Thus, separation of the stereoisomers is required where the ultimate, desired steroid or steroid derivative is one having biological activity.
For example, vitamin D.sub.3 is a steroid derivative which may be synthetically prepared from precursors leading to 7-dehydrocholesterol, which is then irradiated by ultraviolet light to produce vitamin D.sub.3. However, precursors for vitamin D must have the side chains thereof set so as to define an R stereo configuration in order to be biologically active. Thus, most of the biologically active steroids and their derivatives have continued to be derived from purification of natural sources.